The present invention relates generally to a digital control system for an automotive hydraulic brake system which prevents the vehicle wheels from locking up or skidding. More particularly, the invention relates to an anti-skid brake control system having a plurality of microprocessors which can operate independently to independently perform anti-skid control for corresponding vehicle wheel or wheels.
Various kinds of anti-skid automotive brake control systems are now available. Anti-skid brake control systems generally control braking pressure at wheel cylinder or wheel cylinders in order to hold wheel slip relative to vehicle speed as close as possible to an optimal rate. In order to accomplish this, various approaches have been taken in the prior art.
U.S. Pat. No. 4,315,213, issued on Feb. 9, 1982 to Manfred WOLFF, discloses a method for obtaining an acceleration or deceleration signal from a signal proportional to speed and apparatus therefor. The method for obtaining an acceleration or deceleration signal from a signal proportional to the speed consists of storing the n most recently ascertained changes in the speed signal in a memory, and upon ascertainment of a new change to be stored in memory, erasing the change which has been stored the longest, and forming a deceleration or acceleration signal by addition of the stored n changes periodically at intervals of dT. In this method, the occurrence of deceleration or acceleration exceeding the threshold is recognized quickly.
U.S. Pat. No. 4,267,575, issued on May 12, 1981 to Peter BOUNDS, discloses a system, which serves to provide signals to a microcomputer-based control system from which instantaneous values of speed can be computed, includes a wheel-driven alternator which provides an alternating current output whose frequency varies with wheel speed. A signal processor converts this signal to a series of sensor pulses whose width varies inversely with frequency. A sample pulse supplied by a microprocessor sets the period or length of time during which the sensor pulses are examined for each speed calculation cycle of the microprocessor. The sample period pulses are AND-gated with a high-frequency clock signal and also with the sensor pulses to provide a series of marker pulses marking the up and down excursions of the sensor pulses. The marker pulses occurring in each sample period are counted directly in a first counter, and in addition are supplied to a latch circuit and from thence to an AND gate which responds to the first marker pulse in the sample period to count occurrences of the first counter exceeding its capacity. A third counter is also connected to receive the high-frequency clock pulses and counts only the clock pulses occurring after the last marker pulse in the sample period. At the end of the sample period, the counts from all three counters are transferred to the microprocessor which uses this information to compute a value for wheel velocity over the sample period. The system continuously provides the input counts to enable the microprocessor to calculate wheel velocity over each sample period.
In another approach, U.S. Pat. No. 4,384,330 to Toshiro MATSUDA, issued on May 17, 1983 discloses a brake control system for controlling application and release of brake pressure in order to prevent the vehicle from skidding. The system includes a sensing circuit for determining wheel rotation speed, a deceleration detecting circuit for determining the deceleration rate of the wheel and generating a signal when the determined deceleration rate becomes equal to or greater than a predetermined value, a target wheel speed circuit for determining a target wheel speed based on the wheel rotation speed and operative in response to detection of a peak in the coefficient of friction between the vehicle wheel and the road surface, and a control circuit for controlling application and release of brake fluid pressure to wheel cylinders for controlling the wheel deceleration rate. The wheel rotation speed sensing circuit detects the angular velocity of the wheel to produce alternating current sensor signal having a frequency corresponding to the wheel rotation speed. The wheel rotation speed sensor signal value is differentiated to derive the deceleration rate.
Another approach for deriving acceleration has been disclosed in U.S. Pat. No. 3,943,345 issued on Mar. 9, 1976 to Noriyoshi ANDO et al. The system disclosed includes a first counter for counting the number of pulse signals corresponding to the rotational speed of a rotating body, a second counter for counting the number of pulses after the first counter stops counting, and a control circuit for generating an output signal corresponding to the difference between the counts of the first and second counters.
In the present invention, another approach has been taken to derive the wheel rotation speed which will be hereafter referred to as "wheel speed" based on input time data representative of the times at which wheel speed sensor signal pulses are produced. For instance, by latching a timer signal value in response to the leading edge of each sensor signal pulse, the intervals between occurrences of the sensor signal pulses can be measured. The intervals between occurrences of the sensor signal pulses are inversely proportional to the rotation speed of the wheel. Therefore, wheel speed can be derived by finding the reciprocal of the measured intervals. In addition, wheel acceleration and deceleration can be obtained by comparing successive intervals and dividing the obtained difference between intervals by the period of time over which the sensor signals were sampled.
To perform this procedure, it is essential to record the input timing in response to every sensor signal pulse. A difficulty is encountered due to significant variations in the sensor signal intervals according to significant variations in the vehicle speed. In recent years, modern vehicles can be driven at speeds in the range of about 0 km to 300 km. Sensor signal intervals vary in accordance with this wide speed range. In particular, when the vehicle is moving at a relatively high speed, the input intervals of the sensor signal pulses may be too short for the anti-skid control system to resolve. As accurate sampling of input timing is essential for the proposed approach, errors in the recorded input time data will cause errors or malfunction of the anti-skid brake control system. One possible source of error in sampling the input timing is accidentally missing one or more sensor signal pulses. Such errors are particularly likely to occur when the vehicle and wheel speeds are relatively high and therefore the intervals between adjacent sensor signal pulses are quite short.
U.S. Pat. No. 4,408,290, issued on Oct. 4, 1983 to the common invertor of this invention is intended to perform the foregoing input time data sampling for use in calculation of acceleration and deceleration. In the disclosure of the applicant's prior invention, an acceleration sensor acts on the variable-frequency pulses of a speed sensor signal to recognize any variation of the pulse period thereof and to produce an output indicative of the magnitude of the detected variation to within a fixed degree of accuracy. The durations of groups of pulses are held to within a fixed range by adjusting the number of pulses in each group. The duration of groups of pulses are measured with reference to a fixed-frequency clock pulse signal and the measurement periods of successive groups of equal numbers of pulses are compared. If the difference between pulse group periods is zero or less than a predetermined value, the number of pulses in each group is increased in order to increase the total number of clock pulses during the measurement interval. The number of pulses per group is increased until the difference between measured periods exceeds the predetermined value or until the number of pulses per group reaches a predetermined maximum. Acceleration data calculation and memory control procedure are designed to take into account the variation of the number of pulse per group.
In such anti-skid brake control systems, it is essential to provide accurate wheel speed sensor signals as all of the control operations are performed in accordance with parameters derived from the wheel speed sensor signals. On the other hand, since the wheel speed sensor is usually installed opposite the vehicle wheel in order to detect the rotation speed thereof, it is susceptible to damage during operation and the harness or wiring connecting the wheel speed sensor to the control module may possibly be damaged. If the control module fails to receive wheel speed sensor signals, it becomes impossible to perform anti-skid control. In the worst case, when the wheel speed sensor fails while the anti-skid control system is in release mode, the pressure control valve may remain in release mode indefinitely, thus supplying a reduced or negligible fluid pressure to the wheel cylinder.
Therefore, for safety, it is essential to provide a fail-safe system or back-up system which can perform braking even when the wheel speed sensor fails.